U.S. Pat. No. 4,178,609 issued 11 Dec. 1979 to R. C. Beutel and entitled "COMB FILTER HAVING IMPROVED LUMINANCE RESPONSE" describes an adaptive interline comb filter used for separating luma and chroma from each other in respective responses to a composite video signal. A detector is used with the comb filter to detects the lack of correlation in successive lines of chroma, responsive to which the operation of the comb filter is changed so as to reduce uncancelled chroma in the separated luma.
U.S. Pat. No. 4,498,099 issued 5 Feb. 1985 to D. H. Pritchard and entitled "APPARATUS FOR FRAME-TO-FRAME COMB FILTERING COMPOSITE TV SIGNAL" describes reduction of objectionable double images in the region of moving edges. During times when moving edges are detected, the chroma signal separated by frame combing is lowpass filtered and combined with the luma signal separated by frame combing, thereby to provide a better separated luma signal to replace the separated luma signal obtained simply by frame combing.
U.S. Pat. No. 4,498,100 issued 5 Feb. 1985 to R. M. Bunting and A. A. Acampora and entitled "APPARATUS FOR FRAME-TO-FRAME COMB FILTERING COMPOSITE TV SIGNAL" describes reduction of the color distortions around the periphery of reproduced color images. During times when moving edges are detected, the chroma signal separated by frame combing is added to the luma signal separated by frame combing and the result is bandpass filtered to provide a better separated chroma signal to replace the separated chroma signal obtained simply by frame combing.
U.S. Pat. No. 4,819,062 issued 4 Apr. 1989 to Song Dongil and Kim Byung-Jin and entitled "ADAPTIVE-TYPE LUMINANCE/COLOR SIGNAL SEPARATION CIRCUIT USING THE HORIZONTAL AND VERTICAL CORRELATION IN TELEVISION RECEIVER" describes adaptive comb filtering that avoids the need for frame-to-frame comb filtering. The specification and drawing of U.S. Pat. No. 4,819,062 is incorporated herein by reference. U.S. Pat. No. 4,819,062 describes a first filter means 25 of a type television engineers sometimes refer to as a "highpass vertical comb filter", a second filter means 26 of a type television engineers sometimes refer to as a "highpass horizontal comb filter," a third filter means 27 of a type television engineers sometimes refer to as a "lowpass vertical comb filter," and a fourth filter means 28 of a type television engineers sometimes refer to as a "lowpass horizontal comb filter." Comb filters have alternating passbands and stopbands across the spatial-frequency spectrum. Ideally, the frequency spectrum of a "highpass" comb filter consists of isolated single frequencies recurring on even interval, and such frequency spectrum is not generated by performing a lowpass-to-highpass transform on the frequency spectrum of a "lowpass comb filter" consisting of isolated single frequencies recurring on even interval, including zero frequency. The frequency spectrum of a "highpass" comb filter can be generated by performing a lowpass-to-bandpass transform on the frequency spectrum of a "lowpass comb filter"; however, selecting the carrier frequency to be midway between two tooth frequencies of the "lowpass comb filter." So "highpass comb filters" are sometimes referred to as being "bandpass comb filters," rather than "highpass comb filters." This latter usage is employed in this specification.
U.S. Pat. No. 4,819,062 describes the use of lowpass and bandpass vertical comb filters 25, 27 for separating luminance and chrominance signals from the NTSC composite video signal when that signal is descriptive of an edge as would be encountered during a horizontal sweep through the image field. This patent describes the use of lowpass and bandpass horizontal comb filters 26, 28 for separating luminance and chrominance signals from the NTSC composite video signal when that signal is descriptive of an edge as would be encountered during a vertical sweep through the image field. The determination of which method is to be selected for separating luminance and chrominance signals from the NTSC composite video signal is responsive to a comparison made between the response of a vertical scan edge detector 34 and the response of a horizontal scan edge detector 35. The horizontal scan edge detector 35 determines the correlation of the composite video signal in the horizontal direction, and the vertical scan edge detector 34 determines the correlation of the composite video signal in the vertical direction.
U.S. Pat. No. 4,819,062 further describes the alternative usage of two types of peaking filters for edges. When the luminance and chrominance signals are separated from the NTSC composite video signal using lowpass and bandpass horizontal comb filters 26 and 28, the vertical high frequencies are separated from the response of the bandpass vertical comb filter 25 with a lowpass filter 40. When the luminance and chrominance signals are separated from the NTSC composite video signal using lowpass and bandpass vertical comb filters 25 and 27, the horizontal high frequencies are separated from the response of the lowpass vertical comb filter 27 with a further bandpass filter 50.
U.S. patent application Ser. No. 07/562,907, filed 6 Aug. 1990 by Christopher H. Strolle, Jung-Wan Ko and Raymond A. Schnitzler; entitled "REMOVAL OF THE FOLDING CARRIER AND SIDEBANDS FROM AN UNFOLDED VIDEO SIGNAL"; and assigned to Samsung Electronics Co., Ltd. describes the use of a soft switch or cross-fader for admixing lowpass-vertical-comb-filtered luminance signal with lowpass-horizontal-comb-filtered luminance signal responsive to a comparison made between the response of a horizontal scan edge detector to the luminance signal supplied for adaptive filtering and the response of a vertical scan edge detector to the luminance signal supplied for adaptive filtering. The luminance signal supplied for lowpass vertical comb filtering and lowpass horizontal comb filtering is described as being obtained by unfolding a folded-spectrum luminance signal demodulated from the frequency-modulated luminance carrier of a playback signal retrieved from a VHS video cassette recording.
According to the terminology employed in U.S. patent application Ser. No. 07/562,907 and adopted herein, a horizontal scan encountering an edge between white and black is said to encounter a "horizontal edge". The edge may be a vertical line, but it is called a horizontal edge because it would encountered during a horizontal scan. Similarly, an edge that would be encountered during a vertical scan is called a "vertical edge". The term If "edge" is sometimes referred to in the art as a "discontinuity", "transition", or "detail". A horizontal scan edge detector is an edge detector operative to detect any "horizontal edge"; and a vertical scan edge detector is an edge detector operative to detect any "vertical edge".